1. Field of the Invention
The present invention relates to a method for connecting optical fiber cables which are covered with metal tubes.
2. Description of the Related Arts
An optical fiber cable generally has the length of several hundred meters to ten-odd kilometers and wound on a bobbin, etc. In actual use, in the case where a connection distance is longer than said lengths, optical fiber cables are connected. In the case of metal tube covered optical fiber cables, two optical fiber cables are connected in the following way. Core wire optical fibers are exposed from the covered parts on the respective end parts of the two optical fiber cables to be connected. The end parts of the optical fibers are fused to each other in the state that metal sleeve having an outer diameter close to that of the metal tube is provided on one optical fiber cable side. Then, the metal sleeve is slidingly moved to a position over the metal tubes of the two optical fiber cables. The metal sleeve is caulked when necessary and welding is done between the metal sleeve and both of the metal tubes.
There are various welding methods, of which laser beam welding is usually used in that highly accurate welding is required as the metal tubes are extremely small in diameters and thin.
Welding must be carried out by setting an optimum welding condition and constantly keeping it, since the core wire optical fiber is easily affected by heat generated at the time of welding the metal tube and the metal sleeve.
The parts of the optical fibers of the two connected optical fiber cables likely to be affected most by heat during welding are completely covered by the metal tubes and the metal sleeve. Even if welding is done under the condition considered to be optimum, there is no way of inspecting the level of its influence during or after welding. Further, as the connected optical fiber cables are very long, even when abnormal states are found by inspecting the characteristics of the optical fibers on the end parts, it is impossible to identify whether the abnormalities have been generated due to fusing of the optical fibers themselves, or influence of heat by welding the metal tubes and the metal sleeve, or changes of the characteristics of the optical fibers on the other parts.
As described above, the two optical fiber cables must be connected by setting the optimum welding condition and doing welding work under such a condition. However, in the conventional manner it has been impossible to find any changes in the condition after setting, and thus connecting work had to be carried on assuming that specified accuracy and characteristics were provided.